Exploring Structure–Property Relations of B,S-Doped Polycyclic Aromatic Hydrocarbons through the Trinity of Synthesis, Spectroscopy, and Theory

Polycyclic aromatic hydrocarbons (PAHs) are prominent
lead structures
for organic optoelectronic materials. This work describes the synthesis
of three B,S-doped PAHs with heptacene-type scaffolds via nucleophilic
aromatic substitution reactions between fluorinated arylborane precursors
and 1,2-(Me3SiS)2C6H4/1,8-diazabicyclo­[5.4.0]­undec-7-ene
(72–92% yield). All compounds contain tricoordinate B atoms
at their 7,16-positions, kinetically protected by mesityl (Mes) substituents.
PAHs 1/2 feature two/four S atoms at their
5,18-/5,9,14,18-positions; PAH 3 is a 6,8,15,17-tetrafluoro
derivative of 2. For comparison, we also prepared the
skewed naphtho­[2,3-c]­pentaphene-type isomer 4. The simultaneous presence of electron-accepting B atoms
and electron-donating S atoms results in a redox-ambiphilic behavior;
the radical cations [1•]+ and [2•]+ were characterized by electron paramagnetic resonance
spectroscopy. Several low-lying charge-transfer states exist, some
of which (especially S-to-B and Mes-to-B transitions) compete on the
excited-state potential-energy surface. Consistent with the calculated
state characters and oscillator strengths, this competition results
in a spread of fluorescence quantum yields (2–27%). The optoelectronic
properties of 1 change drastically upon addition of Ag+ ions: while the color of 1 in CH2Cl2 changes bathochromically from yellow to red (λmax from 463 to 486 nm; −0.13 eV), the emission band
shifts hypsochromically from 606 to 545 nm (+0.23 eV), and the fluorescence
quantum yield increases from 12 to 43%. According to titration experiments,
higher order adducts [Agn1m]n+ are
formed. As a suitable system for modeling Ag+ complexation,
our calculations predict a dimer structure (n = m = 2) with Ag2S4 core, approximately
linear S–Ag–S fragments, and Ag–Ag interaction.
The computed optoelectronic properties of [Ag212]2+ agree well with the experimentally observed
ones.